1. Field of the Invention
The present invention relates to a method of forming a fine pattern with an electron beam lithography technique to produce a semiconductor device or an integrated circuit.
2. Description of the Prior Art
In the manufacturing process of an integrated circuit or the like, patterning is performed in the photolithography technique with use of an ultraviolet ray. As the size of an element has been demanded to be smaller, the numerical aperture of stepper lens has been increased and a light source of shorter wavelength has been used. However, this makes the focus depth shallow. On the other hand, as the pattern size of an LSI element has become finer and an ASIC has been manufactured, an electron beam lithography has been developed.
An electron beam resist is indispensable for the formation of fine pattern with the electron beam lithography technique. Among the electron beam resists, polymethylmethacrylate (PMMA) is known to have the highest resolution, but its low sensitivity is a problem. Then, recently a lot of workers developed many resists to enhance the sensitivity of a positive type electron beam resist: polybutyl methacrylate, a copolymer of methyl methacrylate and methacrylic acid, a copolymer of methacrylic acid and acrylonitrile, a copolymer of methylmethacrylate and isobutylene, polybutene-1-sulfone, polyisopropenyl ketone, and fluoropolymethacrylate. These resists all have an electron withdrawing group in a side chain or introduce in a principle chain a bond which can be decomposed easily with an electron beam and to enhance the sensitivity. However, they do not satisfy both resolution and sensitivity. Further, the dry etch resistance and the heat resistance are not good enough. Therefore, they are hardly used for a mask for dry etch, and its use is limited.
On the other hand, when a positive type resist having a one-component polymer based on PMMA is developed, an organic solvent is necessary, and when the development is performed, the resist film may swell in the developer with an organic solvent. Then, the resolution of the pattern becomes lower, and the pattern may be distorted in some cases and cannot be used practically. Further, a developer with an organic solvent is harmful on environment and health, and it is also not appropriate as to the flammability.
Recently, enhancing the sensitivity of a positive type electron beam resist has been tried by introducing the concept of chemical amplification. The resist is a three-component substance including a photo acid generator which can generate an acid when irradiated with an electron beam, a polymer which reacts with acid, and a novolac resin as a matrix polymer.
The photo acid generator is for example an organic halide compound or an onium salt. The organic halide compound may be 1,1-bis[p-chlorophenyl]-2,2,2-trichloroethane, 1,1-bis[p-methoxyphenyl]-2,2,2-trichloroethane, 1,1-bis[p-chlorophenyl]-2,2-dichloroethane or 2-chloro-6-(trichloromethyl) pyridine. The onium salt may be a triphenyl sulfonium salt or a diphenyl iodonium salt. These compounds generate a Lewis acid as a strong acid when irradiated with an electron beam.
Some examples of the polymers which react with the acid have the following chemical formulas: ##STR1##
These polymers react with the generated acid according to the following decomposition reaction: ##STR2##
The matrix polymer conducts the following decomposition reaction (hydrolysis reaction ) with an acid generated:
In order to proceed the decomposition (hydrolysis) reaction, a heat treatment is needed just after the drawing of a pattern with the electron beam. Then, the reaction or the decomposition of the side chain proceeds. That is, a Lewis acid is generated when a pattern is drawn with an electron beam in the polymer insoluble in the alkaline solution, and the acid makes the polymer soluble in the alkaline solution. Thus, a positive type pattern can be formed.
Further, in the electron beam lithography, there are problems such as dry etch resistance of electron beam resist, the poor heat resistance, the effect of proximity effect due to the forward and back scatterings of electrons on the precision of pattern. In order to counteract these problems, a multi layer resist which divides the functions of the resist into an imaging layer and a planarizing layer is very favorable.
FIGS. 1(a)-(d) illustrate a prior art three-layer resist process of electron beam lithography. In order to suppress the proximity effect, a high polymer organic film as a bottom layer 51 by 2-3 .mu.m thickness is applied to a substrate 55, and the heat treatment is performed (FIG. 1(a)). Further, an inorganic film of SiO.sub.2 or the like or an inorganic high polymer film of spin-on-glass (SOG) or the like of 0.2 .mu.m as an intermediate film 52 is applied to the bottom layer 51, and an electron beam resist such as PMMA of 0.5 .mu.m thickness as a top layer resist 53 is applied to the intermediate layer 52 (FIG. 1(b)). Then, a pattern is drawn with an electron beam 54 on the resist film 53, and a resist pattern 53P is obtained by developing of a developer of an inorganic solvent (FIG. 1(c)). Next, the dry etching of the intermediate layer 52 is performed with use of the resist pattern 53P as a mask, and the dry etching of the bottom layer 51 is performed with use of the intermediate layer 52 as a mask. Thus, the pattern is transferred (FIG. 1(d)).
A fine pattern of a high aspect ratio can be formed by using a multi-layer resist process. However, the three-layer resist process needs complicated steps and generates many defects. If the selectivity against the etching of the intermediate and bottom layers is small, the pattern size shift on the pattern transfer increases by 0.1 .mu.m or more. Therefore, the three layer resist is difficult to be used practically.
In the electron beam lithography process, incident electrons are scattered inside the resist, and the electrons which reach the substrate are scattered backward and returned again to sensitize the resist. Because the precision of the pattern worsens largely due to such proximity effect, the bottom layer is required to be applied thickly in order to suppress the backward scattering electrons.
Then, silicone-containing resists and inorganic resists for a bi-layer resist process having two functions as a mask of the bottom film and the resist layer have been developed. For example, a substance including a siloxane bond in the principle chain, a ladder type polysiloxane and a chalcogenide glass type inorganic resist. However, they cannot yet improve the dry etch resistance sufficiently, and the sensitivity and the resolution are too poor to be used practically. In concrete terms, the sensitivity of prior art main resists are about 20 .mu.C/cm.sup.2 and the resolution is about 1 .mu.m. Further, because these resists use an organic solvent as a developer, the changes in size and in sensitivity are large, the process latitude is small, and swelling occurs on developing which makes it difficult to form the pattern correctly. Further, the environment pollution and the harms to the human body are also problems.